Are you saying the manifold is cracked, or that the gasket surfaces are warped?
If cracked, you should give brazing a try. Brazing cracks in cast iron work a lot better and prove more reliable than welding, the flux-coated bronze brazing rods are cheap, and you only need a MAPP or perhaps even propane torch to heat the manifold enough for the brazing medium to penetrate the cracks.
There's a bit of a learning curve to this technique, but if you have a solid grasp on soldering, you can learn to braze.